A printed circuit board (PCB) generally includes a multi-layered board having electronic components, such as chips, transistors, resistors, etc. mounted thereon. Conductive traces formed on the layers of the board connect the electronic components together to form an electronic circuit. The electronic components may be a surface-mount type where the legs of the components are mounted on top of conductive pads. Alternatively, the electronic components may be a through-hole type where legs of the components extend through vias (small holes) within the PCB.
Wave solder machines are used to mount the electronic components to the PCBs. The wave solder machines have multiple stages including a fluxing stage, a preheating stage, and a wave solder stage. The fluxing stage applies flux to the PCB, as further described below. The preheating stage raises the temperature of the PCB in preparation for soldering. And the wave solder stage applies solder to the PCB, thereby bonding the electronic components to the PCB.
Liquid dispensing systems are used during the fluxing stage of the wave solder machine. Before soldering the electronic components to the PCB, flux is applied to remove or facilitate removal of oxides from metal parts, such as the legs of the electronic components. The flux also prevents further oxidation of metals during soldering. Flux applicator systems are a particular type of liquid dispensing system wherein a reciprocating or fixed spray nozzle system ejects atomized flux in a precise pattern over an entire side of the PCB. The flux applicator systems typically are either an air-pressure type or an ultrasonic type. Both types of flux applicator systems have potential problems. The air pressure-type flux applicator systems generally produce a number of relatively large drops of flux traveling at high velocities. At least some of these high-velocity drops bounce off of the PCB, rather than settling on the desired surface. Ultrasonic-type flux applicator systems use high-frequency sound waves created by piezoelectric crystals. Liquid is introduced onto an atomizing surface through a feed tube and is directed toward the PCB by forced air. The vibrational amplitude of the atomizing surface is carefully controlled. If the amplitude is too high, a condition known as cavitation occurs where large droplets of fluid are ejected. If the amplitude is too low, the energy is insufficient to produce atomized drops.
Applying an even distribution of flux is important when producing the PCBs. If too little flux is applied to any portion, oxidation can occur during the soldering process. On the other hand, too much flux requires cleaning of the PCB, which can be expensive and slow the manufacturing process. For example, cleaning a PCB requires expensive cleaning equipment, disposal of chemical waste, and time for drying the PCBs. To ensure that an even distribution of flux is applied to the PCBs, a test PCB is traditionally is sent over the flux applicator system with thermal paper attached to the underside of the board, such as by taping. The thermal paper reacts with the flux sprayed from the flux applicator system causing the thermal paper to discolor. If the thermal paper is not completely discolored, then the flux is not being applied properly. The thermal paper may also be taped or otherwise placed on the topside of the PCB. Alternatively, cardboard may be sent through the flux applicator system to simulate a PCB. The cardboard discolors in spots where the flux is applied. Still further some companies run actual product through the wave solder machine and check if the product was properly produced. This typically requires scraping some of the product as adjustments are made to the flux applicator system.
This prior technique for testing the flux applicator system is problematic. First, the thermal paper can only detect if too little flux is applied, not if too much is applied. Additionally, the thermal paper does not adequately absorb flux well so the flux puddles and often smears before a technician can view an image formed on the thermal paper. Furthermore, the flux may be applied enough to discolor the thermal paper, but not enough to transfer through the vias in the PCB, as is needed to ensure proper soldering.
An object of the invention, therefore, is to provide a method and apparatus for accurately measuring the distribution of liquid dispensed from a liquid dispensing system.